Evolution of new species requires few genetic changes

Oct 31, 2013

These are Heliconius cydno and H. pachinus butterflies. Credit: University of Chicago, Marcus Kronforst

Only a few genetic changes are needed to spur the evolution of new species—even if the original populations are still in contact and exchanging genes. Once started, however, evolutionary divergence evolves rapidly, ultimately leading to fully genetically isolated species, report scientists from the University of Chicago in the Oct 31 Cell Reports.

"Speciation is one of the most fundamental evolutionary processes, but there are still aspects that we do not fully understand, such as how the genome changes as one species splits into two," said Marcus Kronforst, Ph.D., Neubauer Family assistant professor of ecology and evolution, and lead author of the study.

To reveal genetic differences critical for speciation, Kronforst and his team analyzed the genomes of two closely related butterfly species, Heliconius cydno and H. pachinus, which only recently diverged. Occupying similar ecological habitats and able to interbreed, these butterfly species still undergo a small amount of genetic exchange.

The researchers found that this regular gene flow mutes genetic variants unimportant to speciation—allowing them to identify key genetic areas affected by natural selection. The butterfly species, they discovered, differed in only 12 small regions of their genomes, while remaining mostly identical throughout the rest. Eight of these coded for wing color patterning, a trait important for mating and avoiding predation, and under intense selection pressure, while the other four remain undescribed.

"These 12 spots appear to only function well in the environment their species occupies, and so are prevented from moving between gene pools, even though other parts of the genomes are swapped back and forth," Kronforst said.

This is a top down view of Heliconius cydno and H. pachinus butterflies. Credit: University of Chicago, Marcus Kronforst

The team also compared the genomes of these two groups to a third species, still closely related but further removed on an evolutionary time scale. Here, they found hundreds of genomic changes, indicating that the rate of genetic divergence accelerated rapidly after the initial changes took hold.

"Our work suggests that a few advantageous mutations are enough to cause a 'tug-of-war' between natural selection and gene flow, which can lead to rapidly diverging genomes," Kronforst said.

The evolution of new species might not be as hard as it seems, even when diverging populations remain in contact and continue to produce offspring. That's the conclusion of studies, reported in the Cell Press journal Cell Reports on Oct. 31, that examine the full genome sequences of 32 Heliconius butterflies from the Central American rain forest, representing five different species. Credit: Marcus Kronforst

Kronforst and his team plan to characterize the remaining four divergent genome areas to look for functions important to speciation. They also are studying why species more commonly arise in tropical areas.

"It is possible that this type of speciation, in which natural selection pushes populations apart, has been important in the evolution of other organisms. It remains to be seen whether it is a common process though," Kronforst said.

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Will someone with graphics capabilities help address this issue by replacing "Hybridization" in frame one of this graphic with "Mutation" and change frame two with a picture of a snake in a tree attempting to ingest a monkey? That would be one way to graphically illustrate for comparison the snake-centric theory of human brain evolution via predation.

Frame four could also be replaced with a picture from what is now central China that compares the physical traits of the adaptively evolved human population that arose during the past 30,000 years to those of any monkeys found in the trees of the same region.

Will someone with graphics capabilities help address this issue by replacing "Hybridization" in frame one of this graphic with "Mutation" and change frame two with a picture of a snake in a tree attempting to ingest a monkey? That would be one way to graphically illustrate for comparison the snake-centric theory of human brain evolution via predation.

Frame four could also be replaced with a picture from what is now central China that compares the physical traits of the adaptively evolved human population that arose during the past 30,000 years to those of any monkeys found in the trees of the same region.

Better, will someone with moderation capability get rid of posts that are off topic?

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